You know its so crazy, I don't like his grooming a*s or anything and I'm defo not rooting for him to end up with the main character. But the younger brother would have been able to maintain his relationship with the MC and his trust if he just literally communicated about his intentions with this marriage and what he aimed for with working together with his fiance, or even telling him about his fiance's girlfriend and how she wants to raise the child with her. Like if he literally didnt treat the MC as an inept little kid who is not a equal partner, he would not have fucked all this up. Again though, eff this creepy a*s guy who basically raised and groomed the MC.